In the manufacture of devices on a semiconductor wafer, it is now the practice to fabricate multiple levels of conductive (typically metal) layers above a substrate. One candidate for on chip multilevel interconnections (both wiring and plugs) is copper, since copper has advantages over other metals, e.g., aluminum and tungsten. However, one of the drawbacks of using copper metallization is its fast diffusion in silicon materials, drift in SiO2 dielectric materials, and diffusion into polymers to form agglomerates. Thus, the implementation of a diffusion barrier is highly desirable and necessary in most instances. A variety of materials are known for forming diffusion barriers on copper. Such materials include, Ta, W, Mo, TiW, TiN, TaN, WN, TiSiN and TaSiN, which can be deposited by physical vapor deposition (PVD) or chemical vapor deposition (CVD). Copper can also be passivated and protected from corrosion by silicide formation in dilute silane, by treatment in 1H-benzotriate, and by trimethylaluminum treatment. Furthermore, Ni, Co and Ni—Co alloys can be electrochemically deposited to serve as a diffusion barrier for Cu metallization. For example, U.S. Pat. No. 5,695,810 to Dubin et al. discloses the use of cobalt tungsten phosphide as a barrier material for copper metallization.
One technique for depositing copper and cobalt, as well as other metals, is electroless deposition. Electroless deposition of metal is a process that involves the formation of a thin film of material from an electrolytic solution or fluid without applying an external voltage to the fluid. The depositing of metal results from the electrochemical reaction between the metal ions of the electrolytic solution, reducing agents, and possibly complexing agents and pH adjusters on a catalytic surface (such as may be found on a semiconductor wafer). Electroless deposition is quite suitable for forming barriers and interconnects between the different layers on a wafer.
A common problem in using baths, which is especially true for the electroless deposition process, is that foreign particles or contaminants can be deposited on the substrate surface of the wafer when transferring the wafers from one bath to another bath. Another common problem is the exposure of the substrate surface of the wafer to air during the transfer (from bath to bath) can cause the non-wetting of deep and narrow trenches in the surface or small via (contact) holes in the surface because of electrolyte evaporation. And yet another common problem is that exposure to air may cause oxidation of the catalytic surface that will result in poor catalytic activity and poor quality metal deposits. This problem becomes especially troublesome when using materials that easily oxidize in air such as copper.
There are three basic types of baths: a full immersion bath, a spray bath, or a combination of the two. A full immersion bath completely immerses a semiconductor wafer in a processing fluid when the wafer is within the bath. The spray bath, on the other hand, uses some type of dispersing apparatus, a spray bar for example, to disperse the processing fluid over the wafer when the wafer is within the bath. A combination bath uses a dispersing apparatus to disperse the processing fluid onto the wafer while filling the bath until the wafer is fully immersed by the fluid.
Immersion plating is limited by the requirement to physically lower the wafer into the plating solution, and remove the wafer after plating. Thus, with full immersion baths and, to some extent, with a combination bath, a time delay is necessary between pre-rinse steps and plating and between plating and post-rinse since the electroless reaction continues in a very uncontrolled fashion while the wafer is lifted out of the solution waits to be rinsed. Moreover, electroless deposition with immersion and using a recirculating system, as disclosed in U.S. Pat. Nos. 5,830,805 or 6,065,424 to Shacham-Diamand et al, will have particles generated in the plating bath due to the presence of the reducing agent in the solution. The particles generated in the recirculated electroless plating bath will be deposited on the surface of the wafer, thereby decreasing yield and resulting in line-to-line shorts or leakage.